Abstract : Relay attacks represent nowadays a critical threat to authentication protocols. They cab be thwarted by deploying distance bounding protocols on an UWB radio. Exploiting the characteristics of time-hopping UWB radios to enhance distance bounding protocols leads to two design strategies. The first one is based on a secret time-hopping code while the mapping code is public. The second strategy exploits a secret mapping code with a public time-hopping code. The merits of each strategy are established over noise-free and noist channels as well as for different radio parameters.